A Fabry-Perot filter (FPF) is an optical device which is constructed to pass light of a selected band of wavelengths. Light entering the filter enters a cavity which is bounded by a pair of reflective surfaces. The reflective surfaces are separated by a precisely controlled distance which determines a set of passbands for the filter. The smaller the separation, the further apart the passbands are in wavelength. That is, the smaller the separation, the larger the free spectral range (FSR) of the filter.
A tunable FPF adds an adjustable component to the separation by which the peak wavelengths of the passbands can be changed. Typically, tuning is achieved in a miniature FPF by making one of the two reflectors a movable or deformable membrane and applying a voltage between the membrane and the second fixed reflector, thereby changing the cavity separation distance through electrostatic attraction. In such a device, the amount of deflection and, therefore, cavity length control, is dependent upon the distance between the reflectors and the level of the applied voltage. For a given starting separation, more deflection requires a higher voltage level; and, likewise, for a given voltage range, more deflection requires that the reflectors be closer together.
At voltage levels compatible with smaller miniature devices, the prior approach to tuning FPFs restricts the device to a relatively small cavity size. This constraint can greatly inhibit the performance of the device by restricting control over the wavelength passbands.
In one aspect, the present invention provides a tunable filter, which in one embodiment is a Fabry-Perot filter (FPF), and a method which overcome these drawbacks of the prior approaches. The FPF of the invention includes an input by which light enters the filter and an output by which filtered light exits the device. A first cavity, e.g., the optical cavity, is provided between the input and the output and is bounded by first and second reflective surfaces. As with the conventional prior art FPF, the wavelengths of light exiting the filter are dependent upon the length of the first cavity. In the present invention, at least one of the reflective surfaces is movable to vary the length of the first cavity to tune the device. The invention also provides a second cavity which is outside the first cavity. A voltage can be applied across the second cavity to move the movable reflective surface to change the length of the first cavity and thereby tune the filter.
In one embodiment, the electrostatic cavity has two electrodes, one fixed and one movable. The movable electrode is coupled to the movable reflective surface. The voltage is applied across the electrostatic cavity via the two electrodes to move the movable surface, thereby changing the length of the first or optical cavity on the opposite side of the movable surface to tune the filter.
One of the reflective surfaces can be curved to present a concave shape to the inside of the first (optical) cavity. The curved reflector can be produced by a mass transport (MT) process of the type described in U.S. Pat. No. 5,618,474, by Liau, et al., issued Apr. 8, 1997, entitled, xe2x80x9cMethod of Forming Curved Surfaces by Etching and Thermal Processing,xe2x80x9d the contents of which are incorporated herein in their entirety by reference. The concave mirror significantly loosens the angular alignment tolerances on the device, and the MT fabrication approach produces an extremely smooth surface, which is beneficial for high optical finesse on the micro-lens-scale concave surface.
The movable reflective surface can be formed as a movable membrane nested within an outer body. The movable membrane is connected to the outer body by a plurality of flexible tethers or flexures. The movable membrane moves axially with respect to the outer body portion via the flexing or deformation of the flexures under the electric field applied across the electrostatic cavity. The flexures are shaped and sized to provide a desired amount of deflection under expected voltage ranges and optical operational characteristics. In one particular embodiment the flexures extend between the movable membrane and the outer body in a straight or radial pattern. In another embodiment, the flexures are formed in a substantially spiral pattern. This latter configuration provides longer flexure length and, therefore, more deflection under applied voltage, while maintaining a relatively small overall surface size.
This reflective surface with the movable membrane, outer body portion and tether pattern can be formed using semiconductor device fabrication techniques. For example, the tether, membrane and body patterns can be defined on a semiconductor layer such as a silicon wafer by photolithography. The patterns can then be formed in the semiconductor by one or more etching steps. The movable membrane can then be at least partially coated with a high reflectivity (HR) coating to provide the desired reflective characteristics for the interior of the optical cavity.
In one embodiment, the filter, e.g., FPF, of the invention is an integrated structure fabricated using semiconductor device fabrication and photolithographic techniques. In one particular embodiment, the device is formed from a silicon-on-insulator device structure.
In the integrated structure of the invention, a first reflective layer and a second reflective layer are formed spaced apart by a spacing layer interposed between them. The thickness of the spacing layer determines the distance between the reflective layers and, therefore, the length of the first, i.e., optical, cavity of the device. At least one of the reflective layers comprises the movable reflective surface noted above which makes the filter tunable. An electrode layer is disposed spaced apart from the movable reflective layer to define the second (electrostatic) cavity outside of the first (optical) cavity. The voltage is applied across the second cavity to move the movable reflective layer.
In one embodiment, the device includes a first electrode coupled to the electrode layer and a second electrode coupled to the movable reflective layer. The voltage used to move the movable layer is applied across the second cavity via the electrodes.
In one embodiment, the spacing layer includes a semiconductor layer, such as a silicon layer. The silicon layer can be sized such as by grinding and polishing to a precise thickness to control the length of the optical cavity of the device. In another embodiment, the spacing layer includes a layer of oxide grown or deposited on a semiconductor layer. The thickness of the oxide can be used to control the length of the cavity. In one embodiment, the spacing layer includes a layer of semiconductor and a layer of oxide, the thickness of either or both of which can be controlled to control the length of the cavity. In another embodiment, the spacing layer comprises a plurality of spacing posts, which can be attached to one or both of the reflective layers. The posts can be made of metal such as gold and can be plated or bonded to one or both of the reflective layers.
The tunable filter and method provide numerous advantages over the approaches of the prior art. For example, as described above, in the present invention, the optical cavity is not the same as the electrostatic cavity. Therefore, the length of the optical cavity can be defined independently of the relationship between the required membrane deflection and the applied deflection voltage. As a result, the cavity can be designed with the freedom and flexibility to meet only specific optical requirements without the electrical constraints introduced in the prior art devices. The filter""s optical parameters, e.g., free spectral range, and the electrostatic tuning parameters can be independently optimized. A much more precise device with more desirable optical performance as well as more efficient electrical performance is obtained.